Method of shaping plastics by extrusion



@6 w i944 J. BAILEY ET AL METHOD OF SHAPING PLASTICS BY EXTRUSION FiledApril 23, 1941 3 SheeiS-Shee; l

me@ 39 3M J. BAILEY ET AL. 2,3537

METHOD OF SHAPING PLASTICS BY EXTRUSION Filed April 23, 1941 3Sheeacs-Shee:l 2

III] me@ E9, M-f. BAILEY ET AL 2.365,3?5

METHOD oF sHAPING PLASTICS BY ExTRUsIoN Filed April 23, 1941 3Sheets-Sheet 5 Patented Dec. 19, 1944 METHOD OF SHAPING PLASTICS BYEXTRUSION James Bailey, West Hartford, and Raymond S. J esionowski,Hartford, Conn., asslg'nors to Plax Corporation, Hartford, Conn., acorporation of Delaware Application April 23,1941, Serial No. 389,912

3 Claims.

This invention relates to the art of shaping organlc plastic. materials.by extrusion thereof through a die to form elongateV or continuousshapes.l Organic plastic materials are referred to herein in the pluralplastics and in the singular as ,plastic.

This application is related to the copending application of JamesBailey, Serial No. 389,911, filed April 23, 1941, in which applicationis disclosed, but not claimed, the invention which is disclosed andclaimed herein.

The copending application referred to describes a method and apparatuswherein and whereby a iilm of liquid medium or lubricant is maintainedbetween the plastic material and the wall or surface of the die throughwhich the plastic material is passed. The advantage of providing such aliquid or lubricant and maintaining it during the extruding operationare that friction, sticking, and other factors tending to prevent orrestrict the passage of the plastic material thr'bugh the die areeliminated or counteracted, so that such passage of the material isgreatly facilitated. This tends to eliminate strains in the extrudedshapes and also permits the use of relatively long dies which are highlyadvantageous because they permit a prolonged temperature treatment whileat the same time a high rate of productionl of the extruded shapes mayThe object of the present invention is not only to obtain advantagesresulting from providing a liquid medium or lubricant in extrusionapparatus but also to provide a novel method for facilitating thestarting operation, for insuring the stability or continuity ofoperation after starting, and also to eliminate jams in the event thatthe plastic material, through inadvertence or some failure in operatingconditions, causes the plastic material to stick in the apparatus in amanner which is not automatically corrected.

Other advantages and objects of the invention will be more fully setforth in the following description which has reference to theaccompanylng drawings depicting the novel apparatus and method and inwhich drawings:

Figures 1a and 1b are more or less diagrammatic views, partly in sectionand partly in elevation,'showing when Fig. 1a is placed at the left ofFig. 1b, extrusion apparatus of improved type embodying the invention,the full length of a Vdie being shown to scale;

Fig. 2 is a partial view in horizontal section on enlarged scale of ascrew stuiler, nozzle, die,

lubricant connections and other parts of the apparatus shown in Fig. 1a;and

Fig. 3 is a view in front elevation showing somewhat more fully theapparatus shown ln Fig. 2.

The invention is of particular utility in shaping by extrusion organicsolvent-free thermoplastic molding material, such asv polymethylmethacrylate and will be described as applied to the production of roundrods of substantial thickness from such material, beginning with thematerial in cold, solid, granular condition, referred to in the art asmolding powder," and ending with the finished product. It is to beunderstood that the invention is not limited to use in shaping suchspecific material nor to forming rods but may be employed in extrudingvarious shapes of 'various organic-therxnoplastic and thermosettingcompounds.

The organic plastic is rst heated as uniformly as possible to soften itto a good working condition, preferably to relatively low viscosity.This operation may advantageously be performed in a heated screw stufferconnected to the `die through which the heat-softened plastic is forcedunder pressure continuously applied to the plastic material to force itthrough, and impart to it the shape of, the die.

A suitable lubricant is selected for use in lubrieating the die throughwhich the selected plastic -is to be forced and various types oflubricants may be employed. An example of the type of lubricant which Iprefer to use in extruding pol"- methyl methacrylate is hypoid gear oil,or oil of the type used in differential gear boxes of automobiles. Thisis a mineral oil. The oil or other lubricant should be stable and shouldnot body up or break down at the highest temperature of the plastic whenin contact with the lubricant. Such oil may have a viscosity of 1100 to3000 seconds at 375 F. in order to insure that the lubricant will retaingood lubricating properties under high temperature and pressure. Thisoil gives a better finish on the extruded shape than most otherlubricants. The lubricant employed, whether it be oil or otherlubricating material, should be immiscible with and inert toward theplastic under the conditions prevailing in the die, although a slightsuperflcial absorption of the lubricant by the plastic material ispremissible in some cases. If objectionable, the supercial layer on theextruded shape which may contain lubricant may be removed by a polishingoperation. The above type of lubricant gives good results in extendingboth polymethyl methacrylate and polystyrene shapes and also otherplastics, such as cellulose acetate. Other examples of lubricants areglycerine mixed with metallic soap, and mixtures of glycerine anddextrine. If the lubricant is too viscous, it may be heated tofacilitate its ow into the die.

In order to achieve good results, a continuous film of lubricant ismaintained on the die, that is, a lm which is not broken through at anypoint between the plastic and the die surface. This usually requiresthat the iilm be maintained at all possible points of contact betweenthe plastic and the die. If this is not done the plastic will touch thedie surface and stick thereto causing an increase in resistance to theflow of plastic through the die followed by an increase of pressure onthe plastic by the stuffer, which is apt to spread the area of stickinguntil the plastic is jammed in the die. A very slightpenetration orpuncture of the film may start jamming under the usual conditions ofoperation.

To insure that the continuous lm will be main.. tained, it is necessarythat the lubricant be supplied under very high pressure. However, thisimmediately leads to difiiculty because unless the pressure is correctwith respect to the pressure in the plastic, either the plastic willdisplace the lubricant and sticking will result or the lubricant willdisplace the plastic and deform it and might even cut off the plasticwhere the plastic enters the die. There is always a danger of theplastic closing olf the inlet or inlets through which the lubricantflows into the die and conversely'there is a danger of the lubricantflowing backwardly through the plastic into the stuer. When that happensthe batch of plastic in the apparatus is spoiled and must be discarded.

To provide the necessary control for maintaining a lrn of lubricant asstated above, the lubricant is conducted fro-m a source of high pressureand admitted to the die in a quantity sufficient to partially fill thedie after which plastic is delivered into the die and forcedtherethrough. The lubricant should be supplied under constant pressure.As the plastic passes through the die, a iiow of lubricant is maintainedinto and through the die, the lubricant being caused to enter the diearound a nozzle'from which the plastic material issues into the die, theplastic also being surrounded by the lubricant Where itissues from thenozzle. 1n fact, in starting the operation with the die empty, theplastic should be surrounded throughout its length by a substantialquantity of lubricant, a surplus of which is maintained in the die untila thread or string of plastic material is formed and issues therefrom.But this surplus must not be excessive because that might cut theplastic.

When a proper flow of the plastic material has been established, the owof lubricant may gradually be reduced, lubricant in the die beingdisplaced by the plastic and forced out of the discharge end of the dieuntil finally only a thin film of lubricant remains between the plasticand the die surface. This film may be about .0005 of an inch thick. Thelubricant at the nozzle or jet is forced back by the plastic materialuntil only a small body or bubble remains which surrounds the nozzle andthe plastic issuing therefrom, this body of lubricant being sumcient,however, in normal operation and under the proper conditions of pressureand ow to prevent the plastic material from flowing backwardy around theouter surface of the nozzle and into the inlets for the lubricant.

The lubricant must be supplied under a pressure which is always greaterthan the normal operating pressure on the plastic material and must alsobe supplied under a pressure which is greater than the maximum pressurewhich may be exerted upon the plastic material in the die at any time.In the apparatus illustrated, a pressure of 6,000 to '7,000 pounds persquare inch is maintained. At the same time, however, a fiow oflubricant into and through the die during normal operation must bemaintained but at a critically low rate related to the unit surface areaof the plastic shape extruded per unit of time. If this critical rate offlow is not maintained sticking may result, or the plastic may bedeformed by displacement thereof by an excessive amount of lubricant inthe die. To maintain this critically low rate of flow, it is necesary toprovide special, accurate, throttling or regulating means to insure thatthe lubricant will flow into and through the die at a constant rate ofonly a few drops per minute. i

Under the preferred conditions of operation, the pressure of thelubricant in the die will be responsive to and equal to the pressure onthe plastic within the die. In other words, the lubricant in the die andthe plastic will normally be in pressure equilibrium or hydrostaticbalance. This condition is highly desirable and in fact necessary forstable operation in order to maintain a continuous film during prolongedoperation; furthermore, certain special conditions must be maintained toinsure that this pressure equilibrium if disturbed will instantly berestored. This is explained below with reference to Figs. 2 and 3.

The advantage of maintaining the lubricant in the die in pressureequilibrium or hydrostatic balance with the plastic and in a conditionresponsive to the pressure on and in the plastic material in the die isthat if the film is punctured and local sticking starts at a point onthe die surface, the resulting increase of pressure on the plastic,resulting from increased resistance to ilow, will instantly build up thepressure in the lubricant in the die, thus tending to restore the filmof lubricant at the point or in the area, where it has been punctured.This is an advantage in addition to the advantage of substantiallyreducing friction, :preventing sticking and counterbalancing the radialcomponents of pressure exerted through the plastic against the wall ofthe die.

Even though the above precautions are taken to prevent sticking of theplastic to the die and to relieve incipient' sticking and the resultingjams which might otherwise occur, occasionally the plastic sticks in amanner which is not automatically corrected, as explained above, and ajam results. In this event, the plastic may be dislodged by promptlycutting oil the flow of plastic into the die, cutting off the flow oflubricant into the die, and permitting the plastic in the die to cooland shrink, If a bad hang-up is to be avoided, plastic and lubricantmust both be cut olf when sticking occurs to remove pressure from theplastic in the die. This prevents spread of the sticking area. After theplastic has been permitted to shrink without pressure thereon, thelubricant is then suddenly admitted at extremely high pressure of theorder stated above so as to deliver a, fluid pressure blow to theplastic in the die and thus dislodge it and break the Jam. If the jam isnot broken on the first attempt, the plastic material is allowed to coolfurther and lubricant is again admitted suddenly under high pressure tobreak the iam.

asoman Because of the fact that the plastic shape slides through the diein or on a nlm of lubricant in pressure equilibrium with the plastic,stressing and resulting strains are greatly reduced. In prior methods,when the hot plastic flows into the die. it stiilens and adheres to thedie surface and succeeding plastic must be forced through this stiffenedplastic throughout the entire length of the die in order to fill the dieand compensate shrinkage. 'I'his results in shearing stresses andresulting strains. Furthermore, in prior methods the plastic swellssomewhat where it leaves the die and tends to become spherical. On theother hand,ln the present method, as the plastic flows into the die andstiffens, it slides bodily through. the die instead of adhering to theSurface of the die. Shearing stresses are largely avoided because thereis very little flow of the hot incoming plastic through the outerchilled plastic already in the die; this occurs only in a relativelyshort zone near the entrance to the die where the plastic is relativelyhot and stresses are at a minimum. Beyond this zone, shrinkage iscompensated by longitudinally squeezing the shape by back pressurethereon.

Furthermore in the present invention, there is no appreciable swellingwhere the plastic issues from the die as in prior methods. Therefore,the extruded shape corresponds to the shape and dimore water sprays.Such cooling also must be gradual in order to prevent vacuum bubbles. Itis preferred to hold the shape straight as it` is cooled or hardened andto support it at spaced points as it travels to prevent deforming ornattening it. The shape may be cut into desired lengths. A

Referring now to Figs. 1a and lb, there is shown a heat-Jacketed stuiier4U of the screw type into which molding compound is introduced in solidcondition and is heated and worked until it is softened to the desiredworking condition. From the stuer 40 the plastic flows through a.homogenizer and filter 4I, thence through a plastic valve 42 and intoand through a nozzle 43 surrounded by lubricant admitted through passage44 from a source and through connections which will later be described.Ihe lubricant thus supplied surrounds the plastic where it issues fromthe nozzle and flows through the die 45 in the form of a very thin filmon the surface of the die and surrounding the plastic material which isshaped into a rod 46 during its passage through the die.

The die illustrated in Figs. la and 1b is approximately ten feet threeinches long, this length being used advantageously for producing the rod46 which, as shown, is intended to be in thickness. The end of the diecarries a stuiiing mensions of the die within close tolerances, box 4lwhich may be tightened more or less to allowing for such shrinkage asresults from cooling of the shape after extrusion.

Sulcient back pressure may be maintained in carrying out my novel methodto insure that gas apply the desired back pressure to the rod duringextrusion thereof and which serves as a very eilicient means to removethe lubricant from the surface of the rod just before it issues from thebubbles and vacuum bubbles are prevented, asdie. A small opening isdrilled in the die or in suming that a proper temperature treatment isapplied to the plastic. Back pressure should be 150 pounds or more persquare inch. Gas bubbles may be caused by volatiles, such as air,solvents,

the gland nut of the stuffing box to permit drainage of lubricant whichmay be caught in a pan, as shown at 48. This prevents deformation of therod 48 which otherwise might result from acor gases given off by theplastic resin or compound cumulation of a surplus amount of lubricantitself. To prevent gas bubbles, the plastic must be held under apressure greater than the maximum vapor pressure and the temperature ofthe plastic reduced below a certain temperature critical for theparticular plastic and held below this temperature for a certain lengthof time.

To prevent vacuum bubbles, a substantial temperature difference betweenthe exterior and interior of the plastic shape must be avoided.Otherwise the exterior will harden under compression, thereby setting uptension on the interior which ruptures the plastic, thus forming" vacuumbubbles. Prevention of vacuum bubbles requires a long temperaturegradient, that is, gradual temperature reduction and the thicker theshape the longer the gradient for the same plastic. Therefore, toprevent both gas and vacuum bubbles, the plastic must be held underpressure and cooled for sufficient time and to do this in the die andproduce relatively thick shapes at practicable speeds, a long die mustbe used. The longer the die, the greater the speed at which the shapemay be extruded.

Although the invention permits the use of a die of almost unlimitedlength, a substantial part of the temperature treatment may convenientlybe performed after the shape has left the die. In shapingthermoplastics, it is preferred to extrude the shape with sumcient heatcontent to reheat the surface of the shape in the atmosphere. 'I'hisusually results in a lustrous surface on the shape. The shape may thenbe permitted to cool by exposure to room or plant atmosphere with orwithout forced cooling, such as provided by one or within the die,especially near its discharge end.

The lubricant is supplied to passage 44 under a. constant high pressureof about 6,000 pounds per square inch by an accumulator 5I, which ischarged at intervals with lubricant by a pump 52 which draws lubricantfrom a suitable receptacle, not shown, and discharges it through aconduit 53 into the accumulator. The accumulator supplies the lubricantunder constant pressure and prevents deformation of the plastic shapewhich might otherwise result from the pulsating effect of the pumpacting on the plastic through the lubricant leading into the die andsurrounding the plastic.

The piston 54 of the accumulator is raised against the weight 55 untilthe accumulator is lled. From the accumulator the lubricant flows underconstant pressure of weight 55 through pipe 56 past a main valve 51through filter 58 and throttle 59, thence through a short pipe section6l into the passage 44. The line 56 is provided with a drain 62 and isconnected to a bypass line 56a controlled by a by-pass valve 63, line56a joining the short connection 6i and serving to by-pass lubricantaround the filter 5B and throttle 59. The details of these parts aredescribed below with reference to Figs. 2 and 3.

It will be understood that during normal operation, the lubricant flowsunder high pressure through the line 56, filter 56, throttle 59 andconnection 6| into passage 44 at a constant minute rate of flow of a fewdrops per minute, deter mined by the adjustment of the throttle 59.However, in starting the operation, the by-pass is used to conduct thelubricant into the die either above or in conjunction with the main line56. The by-pass also is used for breaking jams.

In starting the operation with this apparatus, the plastic valve 42 isclosed, as also is main valve 51 and by-pass valve 63. The stufler isheated and plastic fed into it if necessary to charge it after thestuier has been started. The stuier is allowed to run until well iilledwith plastic and until pressure builds up suiilciently to bring theplastic in good working condition up to the plastic valve ready to owinto the die 45 when valve 42 is opened.

Before plastic is admitted to the die, the die must be cleared, that is,freed of plastic or the plastic therein completely surrounded bylubricant and freely movable under pressure.

A shot of lubricant may now be admitted into the die as by closing drainvalve 62 and opening main valve 51 and by-pass valve 63. The entirevolume of the accumulator may be so admitted. Valve 42 being closed, nolubricant may iiow into the filter or stuier.

Valve 42 is now opened slightly to permit plastic to enter the die 45and may be opened more and more as the now of lubricant is reduced bygradually closing by-pass valve 63, which is so manipulated as to permitthe plastic to form a continuous string and then fill the die, asexplained above. The plastic is admitted gradually at rst to prevent itfrom surging through the die.

As the plastic approaches its full shape, the control of the flow ofllybricant may be taken over by line 56 and throttle 59, the by-passvalve 63 finally being closed. Care must be taken in reducing the iiowof lubricant and increasing the ow of plastic to prevent the plasticfrom displacing the lubricant around the nozzle 43. To guard againstthis, the lubricant should be maintained in a body or bubble ofsubstantial thickness around the plastic for a short distance beyond theend of the nozzle as shown in Fig. 2 and should be brought to thiscondition very gradually by proper manipulation of the valves 42 and 63and by the throttle 56, if it has not previously been set in adjustedposition for normal operation.

In the production of certain shapes, such as rod 1 inch thick, theplastic valve 42 is not opened very widely and thus serves to provideback pressure in the lter and the stuier, thus insuring good density andelimination of air pockets in the plastic. This also protects the lteragainst collapse. In producing smaller shapes, valve 42 is opened fully,the nozzle or jet providing the desired back pressure.

In case of a jam, plastic valve 42 is closed, the stuier is shut oif,main valve 51 is closed and drain 62 is opened. This takes the pressureoff of the plastic in the die, permitting it to shrink out of contactwith it where it stuck in causing the jam. Drain 62 is provided to drainout any leakage which might flow past main valve 51 and accidently exertpressure on the rod in the die. When the plastic has cooled and shrunksomewhat, the jam is broken by rst closing valve 62 and opening valve 51and then suddenly opening by-pass valve 63 to apply a fluid pressureshock to the plastic. This blow is repeated after further. cooling ifnecessary.

Because of the relatively great length of the die 45 compared to itsdiameter. it is conveniently formed in sections instead of in one piece,the die illustrated being formed in three sections, 45a. 45h and 45c,the adjoining ends of which are threaded and Joined by unions i4 and 65.Each section of the die is provided with a jacket. as shown at 66, 66aand 86h, connected at the joints in the die by conduits 66e and 66d.These jackets receive temperature controlling medium to control thetemperature of. the plastic within the die.

Water may conveniently be used for this purpose and in the production ofthermoplastic shapes of substantial thickness it is usually necessary toheat the water in order to prevent cooling the plastic shape tooquickly, which might result in vacuum bubbles in the extruded shape. Inthe apparatus as illustrated in Figs. 1a and 1b, water is drawn from areceptacle 61 through line 68 by a rotary pump indicated at 69, the line68 passing through a heater 1i and thence into jacket 66 at the endthereof where the plastic material enters the die through the nozzle 43.The heated water then ilows through the jackets 65, 86a and 66h, andnally is discharged from the end of the jacket 66h near the dischargeend oi the die into a return pipe 12 which leads into the receptacle 61.

As the water circulates through the jackets of the die it picks up acertain amount of heat from the plastic material and. this heat must bedissipated in order to preventl the temperature of the water from risingabove the desired temperature, This is accomplished by means of a watercooling coil indicated at 13 in the receptacle.

It will be understood that suitable temperature controlling means (notshown) may be associated with the water circulating system for the dieto maintain the desired temperature and temperature gradient in the dieto properly cool the plastic material which is forced therethrough inaccordance with the method of the invention.

To prevent deformation of the plastic shape or rod after it has issuedfrom the die, it preferably is supported by rollers having minimumcontact with the shape or rod. The contact of the rod at any one pointon its surface with any of the rollers is line contact and onlymomentary, and, therefore deformation is prevented, such as would occurfor example if the rod were permitted to rest on a conveyor belt.

the rod 46, although it will be obvious Ithat the rollers may havedifferent forms, depending upon the shapes of the extruded products.

In order to hold the rod straight as it cools, a puller (not shown) maybe provided at the end of the flight of rollers which flight is onlypartially shown. The details of the puller are described Vin thecopending application of James Bailey.

Serial N0. 389,911, led April 23, 1941, and the purpose of the puller isto maintain a tension on the rod and hold it straight as it cools. Thespeed of the puller may be varied in accordance with variations in therate at which the rod is extruded, by means including roller 18 on lever19 of the puller speed control switch 8|, This switch is connected bythe three wires 82 to a speed controller (not shown) for the puller. asis shown in Fig. 1b.

The switch 8l is operated according to the horizontal position of astretch of the rod between rollers i4 and l5 so that when the sag in therod increases the speed of the puller is increased and when the rodapproaches horizontal position the speed of the puller is decreased. Inthis way a substantially constant pull is maintained on the rod,although with some types of rod, or with some types of materials, orwith certain sizes of rod, the puller may be operated at the maximumrate of extrusion and so adjusted that it can slip if the extrusion ratefalls below the maximum rate.

In Fig. 1b, the rod may start to reheat as soon as it issues from thedie and continues to reheat for some distancel say up to the rst rolleror thereabouts. After a certain amount of reheating has been effected,cooling of the rod begins, the exact point where this occurs varyingwith the type of material being shaped and depending upon theestablished conditions of operation.

Certain types of shapes and shapes of certain materials may convenientlybe cooled by one or more water sprays (not shown), so located as todirect streams downwardly over the rod in order to cool it uniformlyover its entire circumference as it passes beneath the respectivesprays.

In some cases the water sprays are not used and the extruded shape iscooled simply by exposing it to room or plant atmosphere.

A cut-off device (not shown) may be provided for cutting the extrudedshape into .suitable lengths. A suitable cut-off device and operatingmechanism associated therewith are described in the co-pendingapplication of James Bailey, Serial No. 389,911, filed April 23, 1941.

The stuffer and associated parts and the connections for applyinglubricant to the die are shown in detail in Figs. 2 and 3. The stuffer40 comprises a heating jacket 40-a for hot oil or Water and contains ascrew 40-b, a drive for which is enclosed in a housing 40c, Fig. 3.

Plastic material ows from the stuffer 40 through nipple 40-d, having aconical inlet and reduced outlet and held in the head of the stuffer bya threaded bushing and screws, as shown in Fig. 2. The nipple 40-d isscrew-threaded into the block 4|-a for the filter 4I' which is held inplace between the head 40-b at one end of the block 4i-a andthe casing42-a of the plastic valve secured to the other end of the block. `Thefilter 4i comprises a metal perforated cylinder wrapped with wire meshcloth preferably of extreme neness, such as 700 wires per linear inch,in order to remove al1 foreign matter, even matter as ne as lint, and toform a smooth colloid by making the plastic material uniform inviscosity. The plastic material is heated to rela.- tively hightemperature and reduced to relatively low viscosity in the stuffer topermit it to flow through such extremely iine screen. The block andfilter are heated by conduction of heat from the stuier.

As shown in Fig. 2, the plastic valve 42 is of the rotary plug type andhas a handle 42-b for turning it. The valve may be securely held inadjusted position by means of a screw 42-c passing through a slot insegment 42-d in the valve Plug. the screw being threaded into a stud42-e secured to the valve block 42-a.

The plastic valve discharges into nozzle 43 projecting into die 45.Nozzle 43 is of small cross section and has a thin wall and is formed ona thick, anged body as shown engaged by a retaining ring 43a fitted ontovalve block 42-a and tted into a member 43-b threaded onto die 45.Member 49-b contains passage 44 for lubricant which flows into thebushing at the rear end of the nozzle into an annular space 44-a betweenthe interior of member 43-b and the nozzle 43. From the rear of thisannular space, the lubricant iiows over the entire ou er surface ofnozzle 43 and over the entire surface of die 45.

The plastic receiving parts between the stuffer and the die aresubjected to very high pressure and are apt to break and plastic is aptto escape through the joints. To avoid breakage. these parts are madewith very heavy walls and leakage of the iiuent plastic is prevented bydrawing the parts together with heavy strain bolts 4Ic and 4l-a'. whichextend through ears in head 4i-b, valve blocks 42-a and 43-b. Byremoving the bolts, the parts may readily be removed for cleaning,repair or replacement.

Passage 44 in bushing 43-b receives lubricant from the short pipe 6lcoupled to nipple 44a, welded to member 43-b as shown in Fig. 2. Pipe6I, nipple 44-a and the block 59-a of throttle 59 have very thick wallsand small bores to prevent spring" in the lubricant connections betweenthe throttle and the die and to keep the volume of lubricant as small aspossible between these points. This insures that a small incompressibleamount of lubricant will be trapped, so to speak, between the throttle59 and the plastic in the die and in an unyielding enclosure in theevent of sudden increase in pressure in the plastic material in the dieresulting from local sticking or other causes. Consequently the pressureof the lubricant will be instantaneously responsive to changes inplastic pressure, insuring continued hydrostatic balance between them.Not only does this tend to prevent or relieve sticking, it also preventsa ilow of the plastic around the nozzle 43 and rearwardly of its tipwhich might cut off the ow of lubricant and cause sticking or jamming.

Throttle 59 comprises a needle 59-b having a sliding fit in its borewhich is moved inwardly and outwardly by turning the hollow handle 59-cin which it is secured at its outer end. The handle 59-c isscrew-threaded on the block 59-a of the throttle. When the needle 59-bis moved into the block. the resistance to flow of lubricant between itand the bore is increased and when moved outwardly, the resistance toflow is decreased. This provides for the minute, critical flow oflubricant required which could not consistently be obtained by the usualtype of needle valve because of abrasion of the seat of such a valve bythe needle engaging it or because of wire drawing of such a valve or forother reasons.

Example I Polymethyl methacrylate molding powder is formed into rod indiameter by heating it in a stuffer and passing it through a lter heldat approximately 415 F. and then discharging the material through anozzle and into a die l0 feet 3 inches long through which lubricant iscaused to ilow in the form of a lm on the die surface at the rate of 20drops per minute under pressure of approximately 6000 pounds per squareinch. A

` temperature of about 146 F. is maintained in the Polymethylmethacrylate molding powder is formed into rod 1" in diameter by heatingthe compound in a stuffer and passing it through a Example IIIPolystyrene molding powder is formed into rod inches in diameter byilrst heating it in a stufi'er and passing it through a illter held at433 F. and discharging the highly heated plastic through a nozzle into adie which is 10 feet 3 inches in length. Lubricant is forced through thedie in the form of a iilm covering its surface at the rate of 12 dropsper minute under a pressure at the source of approximately 6000 pounds.

per square inch. The temperature of the die corresponds to that of ajacket temperature of 171 F. The rod may be so extruded at the rate of10.4 feet per minute, or 33 pounds per hour. The rod thus produced frompolystyrene molding compound has a dull surface.

Example IV Polystyrene molding compound is subjected to heat andpressure in a stuii'er and passed through a filter held at approximately433 F. and then discharged through a nozzle into a die 14 feet 6 incheslong and formed into a rod 1 inch in diameter. The jacket temperature ofthe die at the inlet end of the die may be 186 F. and the ilow oflubricant through the die may be regulated to 11 drops per minute. Therod may be extruded at the rate of 1.7 feet per minute, or 38 pounds perhour.

In Examples I to IV above, a temperature gradient may be maintained inthe die corresponding to a rise of about 10 F. in the jacket water fromthe inlet to the discharge end of the die, a substantial amount of heatbeing transferred from the plastic into the jacket water so that the rodis cooled gradually and substantially hardened in the die. V

Various changes may be made in the details of construction of theapparatus land in the mode of performing the method of the inventionwithout departing from the scope of the appended claims. In addition torods, tubes and ribbons may be made and in making tubes, lubricant maybe applied to either or both sides in the manner disclosed herein. Itwill be understood that in the shaping of thermosetting organicplastics, the temperature of the die is so controlled as to deliver heatto, rather than to remove heat from the plastic material in the die,although some cooling of such thermosetting plastic in the die maydesirably be effected.

In the production of tubes, the lubricant may be introduced between theinterior surface of the plastic material and the exterior of the mandrelover which the tube is formed. For this purpose a circular passage forlubricant is provided at the inner end of the mandrel, or at any otherdesired point thereon.

The condition of the plastic material or batch as supplied to thestutter may vary. In addition to molding powder, batches in more or lessplastic condition, or even in semi-liquid or liquid condition, may befed into the stuer and formed into the desired shapes by the novelmethod of this invention.

Instead of producing bubble-free shapes, the invention may be employedto produce ornamental bubble or foam-containing shapes, such as rods, inwhich gas or vapor bubbles are permitted to form within an exteriorshell of bubblefree or clear material. This may be done by increasingthe rate at which the shape is extruded so that the effect of pressureand temperature change,such as cooling, is insuiilcient to prevent theformation of gas bubbles inwardly of the clear outer layer.

Other changes and variations may be made in practicing the invention.

We claim:

1. In the process of shaping organic plastic material by first placingmaterial in a stuiler and then forcing the material through the stuiIerinto and through a die, the steps which comprise injecting a shot oflubricant into the die at the end thereof into which the materialissues, starting the stuffer and continuing its operation until thepressure of the stuifer on the plastic material has increased to apredetermined amount, then permitting the plastic to flow from thestuiIer into the die while also permitting a reduced ilow of lubricantinto and through the die around the plastic material as the materialissues into the die, this operation being continued until a continuousstream of plastic material has formed in and issues from the die,gradually applying back pressure to the material issuing from the die,and gradually reducing the ow of lubricant through the die until the dieis substantially filled with plastic material surrounded by a. thin lmof lubricant, and finally maintaining the ilow oi lubricant through thedie at a predetermined minute volume per unit of area of the extrudedshape which is extruded from the die per unit oi time.

2. The process of shaping organic plastic material which comprisesextruding said material through a die While maintaining between theplastic material and the die a continuous iilm of lubricant byconstantly forcing the lubricant at a predetermined minute rate andunder pressure into the die and about the material as the latter issuesinto the die and when the plastic material sticks to the die wallcausing a jam of the plastic in the die breaking such jam byinterrupting the fiow of plastic into the die, interrupting the flow oflubricant into the die, shrinking the plastic material and suddenlyadmitting lubricant under extremely high pressure into the die.

3. The process of shaping solvent-free organic plastic material byextrusion through a die of substantial length which comprises coatingthe extrusion die with lubricant, admitting heated plastic to the die ata rate less than the selected normal rate of flow while delivering thelubricant to the die at the end thereof into which the material issuesand at a rate of ilow in excess of the normal rate of iiow until acontinuous stream of plastic is formed in and issues from the die,gradually reducing the rate of ilow of lubricant through the die .to apredetermined minute volume per unit of area o! the shape extruded fromthe die per unit of time while gradually increasing the ow of thematerial to its normal rate.

JAMES BAILEY.

RAYMOND S. JESIONOWSKI.

